The present invention relates to an apparatus for discharging liquid which is provided with a head having a plurality of liquid discharge parts having nozzles collaterally arranged therein, and to a method for discharging liquid employing the apparatus for discharging liquid which is provided with a head having a plurality of liquid discharge parts having nozzles collaterally arranged therein, and in particular, to an apparatus for discharging liquid which has line heads, and to a method for discharging liquid employing the apparatus for discharging liquid which has line heads.
Conventionally, there is known an ink jet liquid discharge apparatus as an apparatus for discharging liquid which is provided with a head having a plurality of liquid discharge parts having nozzles collaterally arranged therein. Such ink jet liquid discharge apparatuses are classified into those of thermal system, those of piezoelectricity system, etc. in view of the difference of liquid discharge manner. Among those, there is widely known a thermal ink jet printer as an apparatus for discharging liquid of thermal system.
As a thermal ink jet printer, there is known a printer which is provided with a printer head having discharge orifices for discharging or spraying liquid ink being liquid in the form of released droplets (referred to also as droplets, hereinafter), ink passes communicating with the discharge orifices, and electro-thermal conversion elements for supplying discharge energy to form droplets which are arranged on the ink passes. The printer employs the serial scan system in which the printer head is scanned or moved along a direction perpendicular to the feed direction of a print paper to print an image.
In the conventional printer, drive pulses are applied to the electro-thermal conversion elements every time the printer head shifts to a print point. As the drive pulses are applied to the electro-thermal conversion elements, liquid ink in the ink passes is supplied with discharge energy, which causes the discharge orifices to discharge the liquid ink in the form of released droplets. Then, an image is printed when thus discharged droplets hit a print paper and form dots.
The printer forms dots on a print paper so that dot matrixes are formed on the print paper when the printer head shifts to a print point. Characters, images, etc. are formed on a print paper using thus formed dot matrixes.
Generally, the printer head used in the printer has a plurality of discharge orifices arranged along a direction (secondary scan direction) perpendicular to the shift direction (main scan direction). In the printer, all the electro-thermal conversion elements can be driven concurrently when printing an image. However, in case all the electro-thermal conversion elements are concurrently driven when printing an image, the burden of a power source unit which supplies power to the printer head is caused to be enlarged. So, time division drive is generally employed in which the plural electro-thermal conversion elements are divided into several blocks, and thus divided electro-thermal conversion elements are sequentially driven every divided block.
On the other hand, a printer generally performs gradation expression under image processing such as the error diffusion method to print an image on a print paper. Generally, a printer employs various image quality modes. For example, a printer employs a mode of printing one line along the main scan direction using one nozzle, and a mode of printing one line along the main scan direction using a plurality of nozzles utilizing the shift operation of a print paper which is fed along the secondary scan direction. When printing an image with high quality, a printer employs the latter mode of printing one line using a plurality of nozzles and reduces shift length of a print paper along the secondary scan direction to perform correction processing so as to make dispersion of hit positions of dots such as banding indistinguishable.
There is a line type printer head which can concurrently print an image along the width direction of a print paper. The line type printer head, being different from a serial type printer head, does not shift along the main scan direction. Instead, the line type printer head itself or a print paper shifts only along the secondary scan direction. Thus, the number of nozzles of the line type printer head along the line direction becomes very large (in 600 dpi pitch, 5100 nozzles for 8.5 inch width). So, the configuration of the line type printer head can be simplified by using head chips in which heaters, etc. for a plurality of nozzles are formed on one semiconductor substrate.
In performing multiple gradation printing, the line type printer head cannot use a printing method used in the serial type printer head. As a printing method of the line type printer head, the PNM (pulse number modulation) system is considered to be effective in which small drop dots composed of a plurality of released droplets repeatedly hit a print paper. However, in case of employing the PNM system, the number of discharge pulses per pixel becomes undesirably large, and “the number of nozzles”×“the number of pulses” is required to be controlled in view of the number of nozzles of the line type printer head, which necessarily and undesirably increases electric power consumption as compared with the serial type printer head.
Furthermore, in performing multiple gradation printing using the line type printer head, since the line type printer head does not shift along the main scan direction, respective nozzles print respective lines. Thus, since the line type printer head cannot use a printing method used in the serial type printer head, printed image is deteriorated due to unevenness, stripes, etc. caused by dispersion of hit positions of dots.
Moreover, in using the line type printer head, since time division drive is employed, discharge timing is undesirably varied. Thus, shear of dot positions is undesirably raised along the main scan direction, which also deteriorates printed image.
The present applicant proposed a method for driving a recording head and a recording head in Japanese Laying-Open Patent 2000-014236, which can reduce shear of dot positions on a print paper as well as lower momentary maximum electric power consumption in performing time division drive. In the Japanese Laying-Open Patent 2000-014236, heater elements are used as drive elements for discharging liquid ink, and a recording head provided with a plurality of recording elements over approximately the width direction of a print paper, which is perpendicular to the feed direction thereof, is used. The plural recording elements are driven under time division drive every constant unit respectively using division drive signals which have their phases shifted for the plural recording elements to cause droplets of liquid ink to hit a print paper. Thus, a plurality of dots are formed on a print paper and an image is printed thereon.
When using head chips in which heaters, etc. for a plurality of nozzles are formed on one semiconductor substrate, dispersion of property cannot be prevented. In case the dispersion of property is large, an image is printed with different density with adjoining parts of adjacent head chips being borders. Thus, in case of printing background using a single color, vertical stripes may be raised at the borders of adjacent head chips along the feed direction of a print paper, which undesirably deteriorates printed image.
The present applicant proposed a printer and a printer head in Japanese Laying-Open Patent 2000-229050 which can prevent deterioration of printed image due to dispersion of property of head chips. In the Japanese Laying-Open Patent 2000-229050, nozzles allocated to head chips are so arranged as to overlap at parts of adjacent head chips when viewed from the feed direction of a print paper. That is, deterioration of printed image due to dispersion of property of head chips can be prevented by employing tiling which has overlapped parts formed therein.
On the other hand, when simply driving line heads employing the tiling which has overlapped parts formed therein under time division drive so as to take advantage of merit of time division drive of line heads and that of the tiling which has overlapped parts formed therein, a problem that coloring of superposed colors at overlapped parts becomes different from that at non-overlapped parts due to difference of degree of dryness is raised.